The present invention relates to the construction of improved electrolytic cells useful as units of a filter press cell arrangement. More particularly, the present invention relates to a unitary frame and membrane member which facilitates cell assembly and membrane replacement. Electrolytic cells are particularly useful in the electrolysis of alkali metal chloride, such as sodium chloride, to produce alkali metal hydroxides, such as sodium hydroxide, together with chlorine and hydrogen.
A filter press arrangement typically consists of a plurality of separate cell units having planar electrode elements generally mounted in a vertical position separated along their active faces by a barrier, such as a diaphragm or membrane layer. The filter press cell units may be monopolar or bipolar and may be appropriately connected in series or parallel to form a cell circuit or bank.
Chlorine and alkali metal hydroxides are essential and large volume commodities as basic industrial chemicals. Plants producing 500 to 1000 tons of chlorine per day are not uncommon. Such plants typically utilize a large number of individual electrolytic cells having current capacities of several hundred thousand amperes. Thus, minor improvements in individual cell operation or performance have major economic benefits because of the volume of the products produced.
Upon the application of direct, electrolyzing current to an electrolytic cell containing an aqueous solution of an alkali metal chloride as the electrolyte, hydrogen and alkali metal hydroxide are produced at the cathode, and chlorine is produced at the anode.
Electrolytic cells that are commonly employed commercially for the conversion of alkali metal halides into alkali metal hydroxides and halides may be considered to fall into the following general types: (1) diaphragm, (2) mercury and (3) membrane cells.
Diaphragm cells utilize one or more diaphragms permeable to the flow of electrolyte solution but impervious to the flow of gas bubbles. The diaphragm separates the cell into two or more compartments. Although diaphragm cells achieve relatively high product per unit floor space, at low energy requirements, and at generally high current efficiency, the alkali metal hydroxide product, or cell liquor, must be concentrated and purified. Such concentration and purification is usually accomplished by a subsequent evaporation step.
Mercury cells typically utilize a moving or flowing bed of mercury as the cathode and produce an alkali metal amalgam in the mercury cathode. Halide gas is produced at the anode. The amalgam is withdrawn from the cell and treated with water to produce a high purity alkali metal hydroxide.
Membrane cells utilize one or more membranes or barriers separating the catholyte and the anolyte compartments. The membranes are permselective, that is, they are selectively permeable to either anions and cations. Generally, the permselective membranes utilized are cationically permselective. Usually, the catholyte product of the membrane cell is relatively high purity alkali metal hydroxide ranging in concentration from about 250 to about 350 grams per liter.
The introduction of dimensionally stable anodes has permitted ever narrowing of the space, or gap, between the electrodes of a cell, thereby facilitating progressively higher cell efficiency. The advent of dimensionally stable anodes and suitable membrane materials has made possible the construction of electrolytic cells having a thin separating partition positioned between planar electrodes, and the combination of a number of individual cell units, usually between about 10 and about 100, to form a cell circuit or bank arranged in the manner of a filter press. For example, in the case of a monopolar arrangement, the components typically would comprise a plurality of anodes mounted in anode frames and cathodes mounted in cathode frames. The anodes and cathodes are separated along their active faces by a permeable barrier, or membrane, and along the inner periphery of the frames by a pliable or elastic gasket member. The assembly is completed by coupling or pressing the components together, hydraulically or by means of threaded connectors, such as tie rods, to compress the gasket members to form gas and liquid-tight seals between the individual units.
The term "membrane", as used herein, is meant to encompass separating partitions or barriers in the form of sheets or fabrics of chemically resistant materials which are ion conducting, for example, permselective resin materials, asbestos fibers, mixtures thereof, and includes microporous materials.
From time to time during the operation of electrolytic cells equipped with a membrane barrier, the membrane component requires replacement. The replacement process typically entails removal of the cell circuit from service, disassembly of the circuit, disassembly of the individual cell unit, removal and replacement of the membrane and the subsequent reassembly of the components into an operative cell circuit. However, the membrane is typically attached to an electrode member, by clamping or other means, and the membrane replacement operation requires the time-consuming steps of removing the usually heavy electrode member, generally by a lifting means, removal of the membrane, positioning and clamping the new membrane on the electrode member and the replacement and repositioning of the electrode member in the cell circuit.
Although the present invention provides a means of shortening the time required to initially assemble a cell circuit, its more important aspect is that the present invention substantially shortens the down time required for replacement of membranes in operating circuits, thereby substantially increasing the production time.